The interaction of endothelial cells in culture with extracellular matrix provides a model for several cell surface-related properties which can be correlated with the synthetic characteristics of these cells, such as maintenance of nonthrombogenic surface, cell polarity, attachment, and morphologic change. Bovine aortic endothelial (BAE) cells secrete fibronectin and type III procollagen, and recently several novel, and possibly unique, endothelial proteins have been partially characterized. These include a pepsin-sensitive collagen, a cell matrix-associated type III collagen which may differ structurally from the secreted form, and two noncollagenous glycoproteins of Mr 170,000 and 40,000 after disulfide bond reduction. Strains of BAE cells have also been described that undergo reversible phenotypic modulation in both protein synthesis and morphology. Collagens and noncollagenous proteins secreted by BAE cells in culture will be characterized further biochemically. We will examine the distribution of these products, using established immunohistochemical techniques, both in vitro and in normal and atherosclerotic tissue. The major secreted and matrix-associated proteins of endothelium from different environments, including bovine cornea, aorta, vein, and capillary, as well as from human endothelial and smooth muscle cells from atherosclerotic lesions, will be identified and compared. Effects on gene expression will be examined under conditions of in vitro senescence and reversible phenotypic modulation by analyzing quantitative changes in the biosynthesis of certain products. The relationship of extracellular matrix to the attachment and growth properties of BAE cells will also be assessed by plating cells on substrates comprised of different collagen types and other endothelial proteins. In this manner we hope to determine the factors that contribute to the maintenance of the normal vascular endothelium and to its repair following injury.